The present invention relates generally to foil bearings used as journal bearings and spring assemblies associated with the foil bearing, and more particularly to a method and apparatus for maintaining a foil journal bearing and its associated spring assembly (if any) in axial position along a shaft within a bore of housing.
Fluid film bearings, also known as foil bearings in the prior art, are used in many diverse applications requiring high speed rotating turbo-machinery. A foil bearing generally comprises two relatively movable elements separated by a thin film of fluid lubricant, such as air, refrigerant, and other such fluids. For example, a foil bearing may comprise a stationary element that surrounds a rotating shaft journal, having predetermined radial clearance therebetween filled with air. The foil bearing may or may not be accompanied by an arcuate corrugated spring to assist in maintaining the optimum geometry of the foil bearing around the shaft.
FIG. 1 shows a cutaway drawing of a typical (one piece) turbo compressor housing 100 with a bore 110 therethrough to receive a rotating shaft (not shown). Midway through the bore 110, a channel 120 may be found which has a slightly larger inner diameter than that of the bore 110. As shown in FIG. 1, the bore 110 is shown as configured for two foil bearings, with each foil bearing being positioned along a region designated herein as a foil bearing surface 130, which comprises an inner wall 140 of the bore. At either end of the bore is located a ring retainer groove 150.
FIG. 2 shows an exploded view of how the shaft, the foil bearing system 240, and the housing 100 interrelate, according to the prior art. The foil bearing system 240 is assembled surrounding the shaft 160 and may be comprised of one or more journal foil bearing assemblies 200 with a sleeve 220 therebetween to hold them in a fixed, spaced apart relationship with each other and with respect to the bore 110. As part of the foil bearing system 240, retaining rings 230 are used on either end of the foil bearing system 240 to hold the bearing assembly in a fixed position within the bore 110 of the housing 100. The foil bearing system 240 is assembled by inserting a retaining ring 230 into a ring retainer groove 150 at one end of the bore 110 in the housing 100. Then a journal foil bearing assembly 200 is inserted against the retaining ring 230, followed by the sleeve 220 and another journal foil bearing assembly 200. When the last journal foil bearing assembly 200 has been inserted, the foil bearing assembly 240 may be held in place by inserting a second retaining ring 230 into the remaining ring retainer groove 150 at the opposite end of the bore 110. The shaft 160 may then be inserted through the foil bearing system 240 and supported thereby.
Referring now to FIG. 3, a cross sectional view of a journal foil bearing assembly 200 is shown. According to the figure, each journal foil bearing assembly 200 comprises one or more springs 202, which in turn may be fabricated from thin corrugated metal sheets with a retaining lug 210 along the sheet. The springs 202 are interposed between the bore and a foil 203 adjacent the shaft 160 and held from contact by the fluid. The foil 203 may also have a downturned retaining lug 210 formed along its extent. For purposes of this disclosure, there is no functional difference between a lug 210 formed in a spring 202 or a foil 203. Therefore, when reference is made to a retaining lug 210 hereafter, the reference should be interpreted as being either for a spring 202, a foil 203, or both. Furthermore, the retaining lug 210 may be formed in a number of ways, i.e. a downturned edge of the spring/foil or a welded bar running axially along the spring/foil intermediate its radial edges. The manner of forming a retaining lug 210 is not relevant to this disclosure and it should encompass any method and manner of providing a retaining lug 210 for preventing radial movement of the spring 202 or foil 203 about the centerline of the bore 110.
Generally, the springs 202 are identical in shape and are fabricated to traverse the circumference of a shaft 160 and occupy the space between the shaft 160 and the inner wall 140 of the bore 110. It should be noted that different applications may require different numbers of springs 202 and foils 203, and some applications may dispense altogether with the springs 202. The configuration shown in FIG. 3 is typical and used to illustrate the general concept only.
Significantly, the foil bearing assembly 200 must be restrained from rotating with the shaft. Therefore, each of the springs 202 and the foils 203 may have a retaining lug 210 formed along its outer surface away from the centerline of the shaft 160 in such a way as to align the spring/foil parallel with a centerline of the shaft 160 and bore 110 surrounding the shaft 160. For this purpose, a number of axial grooves 170 may be machined into the inner wall 140 of the bore 110 such that they are parallel with the centerline. The retaining lug 210 is formed to fit into an axial groove 170 in the inner wall 140 of the bore 110 to prevent the spring 202 or the foil 230 (and thus the journal foil bearing assembly 200) from rotating with the shaft 160 and to maintain its position along the foil bearing surface 130. The axial groove 170 generally runs the extent of the bore 110.
A turbo compressor machine may have one or more journal foil bearing assemblies 200 upon which the shaft 160 rotates. Typically, two such assemblies 200 are configured within the bore 110 of the housing 100. Each journal foil bearing assembly 200 must be held in place along the journals of the shaft 160. Standard retaining rings 230 are used to constrain axial movement of the journal foil bearing assemblies 200 on the outboard ends of the shaft 160, where each retaining ring 230 is held in a retaining ring groove 150 (FIG. 1).
However, on the inboard side of each journal foil bearing assembly 200, installation or use of a retaining ring 230 is difficult due to space limitations, particularly when the housing is fabricated from a single casting. Current practice is to install a single coiled up sleeve 220 fabricated of thin sheet metal in the channel 120 between the two foil bearing surfaces 130 (FIGS. 1 and 2). The sleeve 220 has an axial slit which allows the sleeve 220 to be compressed into a smaller diameter, i.e. a diameter less than that of the bore 110, so that the sleeve 220 can be inserted into the bore 110 and allowed to “spring back” into its original diameter which coincides with the inner diameter of the channel 120.
Referring to FIG. 4, the journal foil bearing assembly 200 is shown as it is held in place by the sleeve 220. According to the figure, the axial groove 170 extends of sufficient depth along the inner wall 140 so that the axial groove 170 opens into the channel 120. This opening allows a sleeve edge 221 to abut a lug edge 212, and thereby prevent the journal foil bearing assembly 200 from drifting inwardly along the shaft 160. The sleeve edge 221 and the lug edge 212 are in approximate 90° relationship to one another and therefore the point of contact is approximately the width of the lug edge 212 and the sleeve edge 221, which is very small in area. Since this point of contact is so small, the sleeve edge 221 and the lug edge 212 can easily cut into or wear into the other.
As can be seen, an improved mechanism is needed to maintain axial separation of the foil bearings along the shaft without excessive wear.